Presumably you want to login, not just start TWS. The easiest way I have found is to do it in Java using 'jemmy' from http://www.netbeans.org. Jemmy is a package for automating Java Swing GUI test harness.

If you need a code fragment I can provide one.
It's only a few lines of code.

It starts TWS and logs in. You need all the appropriate jars in your classpath to run it. It's easier to build the thing in a Netbeans project and have it create a single jar containing jemmy, TWS - the lot.